The present invention relates to a color encoder measurement system which checks whether a color encoder used in a broadcasting station satisfies a standard or not.
The Wireless Telegraphy Act. of Japan requests that a color encoder or the like used in a broadcasting station should be checked whether an output signal therefrom satisfies a standard or not by measuring characteristics thereof every predetermined period. FIG. 12 shows a prior color encoder measurement system used to measure such characteristics.
In FIG. 12, a reference number 10 represents a color encoder as a device under test (DUT) and an input stage thereof is connected to a test signal generator 30 which generates a composite signal as a test signal in general. The color encoder 10 produces at an output stage thereof an output signal (composite television signal) which is applied to a characteristic measurement means 50 in order to measure a frequency characteristic and a delay time difference of the output signal from the color encoder 10. The characteristic measurement means may be a waveform observing apparatus, such as an oscilloscope 50. In this example, the color encoder 10 is for the NTSC signal but it may be for another television system signal.
The color encoder 10 is constructed by considering a practical operation but it includes a circuit to be used only for the measurement by considering the measurement. Input terminals 12r, 12g and 12b receive output component signals (e.g., R, G and B primary color component signals) from a video camera of the broadcasting station at the practical operation and receive a predetermined test signal for the characteristic measurement period. An operation while receiving the test signal will be discussed hereinafter.
The test signal for a predetermined measurement item is applied through at least one of the input terminals 12r, 12g and 12b to a matrix circuit 13 for a matrix arithmetic process. The matrix circuit 13 produces a luminance signal Y, an I-axis signal I and a Q-axis signal Q. Low pass filters 14 and 15 limit the bandwidth of the I-axis signal I and the Q-axis signal Q, and these signals I and Q are applied to a modulator 16 through a delay circuit 27 and directly to produce a color signal C.
The color signal and the luminance signal Y are applied directly and through a timing adjusting delay circuit 26 to a mixer 17 in which these signals are mixed and a well known composite sync signal is added to produce a composite television signal at an output terminal 18.
The basic construction of the color encoder 10 for the practical operation was described hereinbefore. The color encoder 10 further includes the circuit used only for the characteristic measurement. Switching means 22y, 22i and 22q are provided at the output stage of the matrix circuit 13. Each switching means 22i and 22q can select an external test signal (signal Is or Qs corresponding to the I-axis signal I or the Q-axis signal Q). Reference numbers 23i and 23q represent external input terminals. The switching means 22y, 22i and 22q select the output condition in response to external control signals applied to control signal input terminals 24y, 24i and 24q.
In this example, measurement terminals 25i, 25q and 25c are provided in order to measure the I-axis signal I and the Q-axis signal Q having the limited bandwidth from the low pass filters 14 and 15 and also to measure the color signal C.
The characteristics to be measured in such color encoder 10 are the amplitude/frequency characteristics of the luminance signal Y, the I-axis signal I and the Q-axis signal Q and the delay time difference between the luminance signal Y and the I-axis signal I and between the luminance signal Y and the Q-axis signal Q. The test signal is selected by considering the characteristic measurement item. The conventional characteristic measurement requested to select one of the three component input terminals R, G and B proper to each measurement item and to apply the test signal at the selected one to the color encoder 10 rather than applying the signals at all the component input terminals. The selection was done by considering which one of the component signals R, G and B results in the highest output level for each measurement item when the test signal is applied to the component input terminals.
For example, when the amplitude/frequency characteristic of the luminance signal Y is measured, the test signal is applied to the input terminal 12g. When the amplitude/frequency characteristic of the I-axis signal I is measured, the test signal is applied to the input terminal 12r. When the amplitude/frequency characteristic of the Q-axis signal Q is measured, the test signal is applied to the input terminal 12b.
The switching means 22y, 22i and 22q are controlled such that only the matrix output for each measurement item is applied. When the amplitude/frequency characteristic measurement of the luminance signal Y is desired, the switching means 22y is controlled as shown by a dotted line and only the luminance signal Y is outputted from the matrix circuit 13 seemingly. When the I-axis signal I is desired to be measured, the similar control is done wherein the switching means 22y is off, the switching means 22i selects the upper contact and the switching means 22q is at the neutral position. Thus, only the I-axis signal I from the matrix circuit 13 is the measured signal to the color modulator
If the color encoder 10 is constructed to receive the I-axis signal I and the Q-axis signal Q in addition to the component signals R, G and B and the characteristics of the low pass filters 14 and 15 and the color modulator 16 are desired to be measured, the switching means 22y, 22i and 22q are controlled such that the external test signals Is and Qs can be selected.
When the delay time difference between the luminance signal Y and the Q-axis signal Q is measured, a measurement reference should be a reference line L whose level is a half envelop amplitude of leading and trailing edges of the luminance signal Y as shown in FIG. 13. The time difference .DELTA.tf and .DELTA.tb between the Q-axis signal Q and the luminance signal Y are measured at the reference level L of the leading and trailing edges. An average value .DELTA.(=(.DELTA.tf+.DELTA.tb)/2) is obtained as the delay time difference between the luminance signal Y and the Q-axis signal Q.
Such measurement system for the color encoder has the following disadvantages.
(1) The specified signal for the measurement item is selected and the selected test signal is applied to the color encoder 10. The video camera generates all the three component signals R, G and B indeed. Thus, the characteristic is measured in a condition different from the practical operation. Since the test signal for the measurement item is a single composite signal instead of an ideal test signal, a pseudo signal is used as the test signal.
Such measurement cannot obtain the actual characteristic of the color encoder 10 for the measurement item. Since the measurement result is based on the pseudo signal, the characteristic of the practical operation is only estimated.
(2) The switching means in the color encoder 10 are controlled such that unnecessary signals for the measurement item are not outputted. For example, when the amplitude/frequency characteristic of the luminance signal Y is measured, the I-axis signal I and the Q-axis signal Q are controlled not to be produced. If only the luminance signal Y is desired to be generated in the ideal matrix construction, the output levels of the I-axis signal I and the Q-axis signal Q should be zero.
There is a possibility that the matrix circuit 13 is not the same as the construction designed in accordance with a matrix arithmetic. In this instance, if the measurement is not done under the condition that the I-axis signal I and the Q-axis signal Q are generated from the matrix circuit 13, the measurement result is not based on the practical operation of the color encoder 10.
When it is desired to measure a cross talk between the luminance signal Y and the I-axis signal I and between the luminance signal Y and the Q-axis signal Q by such measurement system, the cross talk value may not measured accurately because of the reasons discussed hereinbefore.
(3) It is troublesome to measure the delay time difference between the luminance signal Y and the I-axis signal I and between the luminance signal Y and the Q-axis signal Q because the delay time difference should be measured at both the leading and trailing edges as shown in FIG. 13 and the average should be calculated.
In order to execute this measurement, a considerably complex test signal should be switched and processed because only one test signal is applied to the matrix circuit 13 for the measurement period.
(4) The color encoder 10 as the device under test includes the unnecessary circuits for the practical operation. These circuits are the switching means 22y, 22i and 22q, the input terminals or the like. Since the color encoder 10 needs to include the additional circuits necessary only for the characteristic measurement, it is expensive in cost.
As being discussed hereinbefore, the prior color encoder measurement system is impossible to measure the characteristics at the practical operation and is troublesome to measure them.
Accordingly, there is a need for a color encoder measurement system which is free from the aforementioned disadvantages, is possible to measure the characteristics at the practical operation and is simple in the measurement.